Unit 9: Inheritance and Polymorphism

Unit 8 left a list that could only hold one class of object and student objects that could not be sorted by a field the student chose. This unit pays both. It covers the same code sitting in two classes, extends and what a child gets for free, super and protected, overriding a parent method, overriding toString and equals across a family, calling super.method to add rather than erase, one variable with many types, a list that holds the whole family, a parent nobody should ever build, abstract methods that force every child to answer, interfaces as a promise without the parts, Comparable, and the choice between an abstract class and an interface.

The performance task is The Hierarchy That Earned Itself. The code is one parent type and at least two children, a driver that puts every object into a single ArrayList of the parent type and loops over it once, and at least one place where a child calls super rather than replacing the parent behaviour. The graded centre is a written defense of at least 300 words that applies the is-a and substitution tests to each pair, describes the composition alternative the student rejected in enough detail that a reader could build it, names one thing deliberately not inherited, and justifies whether the parent is abstract, concrete or an interface.

The design decision is assessed as a decision. A correct hierarchy defended only by restating a definition cannot score above the middle band, and a student who concludes honestly that composition would have been better, then shows the composition version, earns full credit on that criterion. A teacher gets a plan and a substitute plan for each of the eighteen lessons, answer keys, tiered handouts, and a sentence frames card for defending a design decision that stays on the desk during the task.

The unit runs 18 instructional days.